Laminations of polyimide films to like films and/or to metal foils

ABSTRACT

A high temperature polyimide film - metal foil laminate and the process for making same is disclosed. The laminate webs are jointed together by an acrylic-epoxy resin adhesive that has been found to be very resistant to delamination of the webs during high temperature applications. The epoxy component of the adhesive is of the low molecular weight epichlorohydrin bisphenol A type having an epoxide equivalent weight of less than 550. The epoxy component is present in a substantial excess in relation to the acrylic component of the adhesive.

United States Patent [191 Sinclair et al.

[54] LAMINATIONS OF POLYIMIDE FILMS TO LIKE FILMS AND/OR TO METAL FOILS[75] Inventors: James R. Sinclair, Charlotte; Gordon Milburn Ellis, Jr.,Matthews,

both of N.C.

[73] Assignee: Rexham Corporation, New York,

[22] Filed: Jan. 13,1971

[21] Appl. No.: 106,251

Related US. Application Data [63] Continuation-impart of Ser. No.868,292, Oct. 27, 1969, abandoned, which is a continuation-in-part ofSer. No. 571,520, Aug. 10, 1966, abandoned.

Hall ..260/837 R IOO PARTS ACRYLIC {60-72 PARTS EPOXY SOLVENT METAL FOIL1 Feb. 20, 1973 2,880,116 3/1959 Alps et al. ..117/72 2,949,445 8/1960Blake ..260/86.l 3,011,909 12/1961 Hartet a1 ..161/186X 3,032,460 5/1962Chipman et a]. .....161/186 X 3,048,552 8/1962 Fang ..260/837 X3,206,330 9/1965 Chottiner ..1 17/218 3,440,215 4/1969 Holub v ..161/227X 3,454,421 7/1969 Westbrook ..161/184 X 3,465,058 9/1969 McCarthy..260/837 R 3,466,347 9/1969 Davis 260/837 R 3,486,934 12/1969 Bond.161/227 X 3,676,566 7/1972 McBride ..161/227 X Primary Examinerl-laro1dAnsher AttorneyMandeville & Schweitzer [57] ABSTRACT A high temperaturepolyimide film metal foil laminate and the process for making same isdisclosed. The laminate webs are jointed together by an acrylicepoxyresin adhesive that has been found to be very resistant to delaminationof the webs during high temperature applications. The epoxy component ofthe adhesive ,is of the low molecular weight epichlorohydrin bisphenol Atype having an epoxide equivalent weight of less than 550. The epoxycomponent is present in a substantial excess in relation to the acryliccomponent of the adhesive.

13 Claims, 2 Drawing Figures PATENTEUFEB201975 3.717. 543

METAL FOIL ido AhTs ACRYLIC 60-72 PARTS EPOXY SOLVENT VENTORS J'AMES vNCLAIR BYGORDON MILBURN ELLISJR I A ORNEY LAMINATIONS OF POLYIMIDE FILMSTO LIKE FILMS AND/OR TO METAL F OILS Related Applications Thisapplication is a continuation-in-part application of our copendingapplication Ser. No. 868,292, filed Oct. 17, 1969, now abandoned, which,in turn, is a continuation-in-part of our application Ser. No. 571,520,filed Aug. 10, 1966, now abandoned.

Background and Summary of the Invention The present invention relates toimprovements in laminations of polyimide film to other polyimide filmand/or to copper and other metal foil substrates; to new and improvedmethods of forming the same; and to improved adhesive compositionsadapted for use in said methods.

Polyimide films have had a known desirability for utilization inconnection with printed circuit laminations and the like because of theinherent high temperature stability of the polyimide material and itsresistance to detrimental environmental conditions. However, itsutilization for these purposes has been significantly restricted becauseof difficulties experienced in fabricating laminations of the polyimidefilm in a manner which would enable the advantageous characteristics ofthe polyimide material to be realized.

Polyimide film material does not melt at any temperature. Accordingly,the initial attempts to laminate the material by thermal techniquesinvolved the production of the film with a combined layer of fiuorinatedethylene polymer resin (du Pont Teflon F EP) to provide a heat sealablesurface. However, the relatively high coefficient of thermal expansionof the Teflon resin and the significant degradation in the strength ofthe Teflon with increasing temperatures, particularly at temperaturesexceeding the fairly low level of about 200F., severly limited theusefulness of the Teflon coated product.

Subsequently, adhesive systems were developed for bonding polyimide filmto polyimide film and/or to metal foils. However, while these knownadhesive systems represented an improvement over the initial Teflonpolyimide approach, the bonding strengths of these adhesives wereinsufficient to utilize fully the properties of the polyimide film sincethese adhesives failed prior to the polyimide film and metal foils.

The present invention is directed to improvements in the laminatingmethods and laminated products of the prior art. The new and improvedpolyimide film lamination of the invention has vastly superiorproperties which make the new laminations especially well suited for thedemanding requirements of sophisticated printed circuit applications.The improvements include the coating of a new and specially formulatedadhesive composition to one web component of the lamination, and thesubsequent formation on the web of a controlled residue of the newcomposition. Thereafter, the webs of the lamination are combined underheat and pressure.

Specifically, and as an important aspect of the invention, the new andimproved adhesive composition is formulated of approximately 100 partsof an acrylic copolymer and 46-100 parts of an epoxy resin component(based on dry solids weights). Most advantageously, a small amount of abrominated epoxy resin is included in the formulation as a portion ofthe epoxy resin component.

Although it has been proposed heretofore to utilize an adhesive mixtureincluding a combination of acrylic adhesive and epoxy resin, no suchadhesive mixture has proved satisfactory for high temperatureapplications. However, it has been discovered, and it is a fundamentalaspect of the invention, that improved laminate bonds are obtained bysubstantially and critically increasing the ratio of the weight of epoxyresin component to the weight of acrylic copolymer component employed inthe adhesive composition, in comparison with the weight ratios of epoxyresin to acrylic copolymer employed heretofore. The increased epoxycomponent of the new adhesive provides a tackiness property to theadhesive residue after substantially all of the solvent is driven off,which tackiness results in a wetting out of the substrate web (aprerequisite for good bond formation) at the subsequent combination ofthe webs to form the new laminate. The wetting out of the substrate web(typically a metal foil such as copper) by the new adhesive aids inestablishing an initial bond between the webs before final cure andresults in a greatly improved laminated material. For example, apolyimide film-metal foil laminated product of the new method is capableof safely withstanding immersion in dip solder baths having temperaturein excess of 500F. for durations of up to 30 seconds, whereas prior artlaminations of this type would be destroyed or severly damaged bysimilar harsh treatment.

Brief Description of the Drawing For a more complete appreciation andunderstanding of the present invention and its attendant advantages,reference should be made to the following detailed description andaccompanying drawing, in which:

FIG. 1 is a schematic representation of the method aspects of theinvention; and

FIG. 2 is an enlarged, fragmentary, cross-sectional view of the new andimproved lamination of the invention.

Description of Preferred Embodiments Referring to FIG. 1, the lamination10 of the invention may be prepared in a continuous process by combiningin a nip 16 a web of polyimide film 11 with a second web 12, which maybe (depending upon the intended end use) either a second web ofpolyimide film or a web of metal foil, such as copper, aluminum,stainless steel, etc. Advantageously for printed circuit base material,rolled copper foil without a surface treatment is utilized for thesecond web 12, while the first web 11 is a so-called H-film, ascommercially available at the time of the filing of this applicationunder the trademark Kapton" from E. I. du Pont de Nemours & Co.,Wilmington, Del. As known to the art, the polyimide film 11 is preparedfrom a condensation reaction between an aromatic tetrabasic acid and anaromatic diamine, and a more complete description thereof is availablein du Ponts Technical Information Bulletin H-l.

As one significant aspect of the invention, a new and improvedlaminating adhesive composition 13 is used to bond the metal foil 12 tothe polyimide film substrate 11. The composition 13 is formulated from100 parts (dry solids basis) of an ammoniated acrylic copolymercomponent, preferably of the methyl methacrylateglycidyl methacrylatetype, and 60-72 parts of epoxy resin component. Most advantageously,about l parts (dry solids basis) of the epoxy resin component of the newadhesive is a brominated high molecular weight epoxy resin.

A series of acrylic copolymers of the methyl methacrylate-glycidylmethacrylate type suitable for use in the adhesive formulation of theinvention are disclosed in the Blake US. Pat. No. 2,949,445, thedisclosure of which is incorporated herein by reference. Acryliccopolymers falling into this group are available from E. l. du Pont deNemours & Co., under the trade designations 6850-6880.

Preferably, the epoxy resin component in the new adhesive formulation isof the epichlorohydrin bisphenol A type, or is a mixture of a number ofresins of that type. Specifically, the epoxy resin component of theadhesive should be a liquid at application temperatures, and it shouldhave a combined epoxide equivalent weight of not more than about 550.The epoxy component may include small amounts of epoxy resins of therequired type having higher equivalent weights as long as the combinedepoxide equivalent weight of the mixture is less than the prescribedvalue of 550.

A preferred epoxy resin for use in the new adhesive, either alone orwith other epoxy resins, is available from the Shell Chemical Companyunder the trademark EPON 828. The resin EPON 828 is a low molecularweight epichlorohydrin bisphenol A type having an epoxide equivalentweight of 190. Preferably, the epoxy component may include up to about30 percent by weight of a brominated high molecular weightepichlorhydrin bisphenol A type having an epoxide equivalent weight ofabout 500. A suitable brominated epoxy resin is available from the DowChemical Company under the trademark DER l l. The inclusion of thebrominated epoxy resin reduces the brittleness of the cured adhesive;however, suitable adhesives for the practice of the invention have beenformulated without its presence. For example, the EPON 828 epoxy resinas the sole ingredient of the epoxy resin component of the adhesive hasproved highly satisfactory.

In accordance with an important aspect of the invention, the epoxy resincomponent of the new adhesive is present in a substantial excess of theminimum amount required for a complete reaction with the acryliccomponent of the adhesive. The invention provides for a ratio of epoxyequivalents to amine equivalents of at least 2.2 to l or greater, whichratio greatly exceeds the stoichiometric requirement for a completereaction between these groups.

As a further aspect of the invention, a preferred acrylic copolymer forpractice of the invention may be synthesized by reacting methylmethacrylate (97.2 parts) with glycidyl methacrylate (2.2 parts) to forma copolymer. The copolymer is then ammoniated with ethanol amine toprovide a curing agent for the epoxy resin ingredients of the newadhesive formulation.

The preferred adhesive formulation of the invention is prepared by firstdissolving about 100 parts (dry solids weight) of the preferredammoniated acrylic copolymer in an appropriate hydrocarbon solvent suchas toluene. About 60 parts of a low molecular weight epichlorohydrinbisphenol A type epoxy resin having an expoxide equivalent of about andI0 parts of a brominated epoxy resin having a bromine content betweenabout 18-20 percent and an epoxide equivalent weight of about 500 arethen dissolved in the solvent mixture.

Referring to the drawing, the acrylic and epoxy components of theadhesive composition 13 are dissolved in the hydrocarbon solvent to forma solvent solution 18. The solvent solution 18 is then applied to thepolyimide film 11 by a reverse roll coater 15 or by any other suitablecoating apparatus such as a gravure coater or a cylinder coater, asshown in H6. 1. The applied coating of adhesive composition is meteredin order to limit the thickness of the residue of adhesive compositionto approximately 0.4 to 1.0 mil upon subsequent solvent removal indryers 17.

After removal of substantially all of the solvent from the adhesivecoating in the dryers 17, the adhesive coated polyimide film 11, and thefoil 12 are subjected to heat (approximately 300400F.) and rollingpressure and are united at a combining nip 16 to establish thelamination l0. Thereafter, the lamination 10 is directed into atreatment oven 19 where it is subjected to a curing cycle ofapproximately 200F. to 260F. for about 16 hours.

An alternative method of producing the lamination l0 utilizes a platenpress in lieu of the combining nip l6 and is characterized as a batchtype rather than a continuous type of operation. The platens of thepress are maintained at approximately 450F., and the strata of thelamination are combined in the press under heat and pressure for 5minutes. The curing cycle necessary in the continuous process iseliminated when the invention is practiced in this manner.

In accordance with the invention, the excess of epoxy resin component(beyond the stoichiometric minimum) of the new formulation functions toprovide a tackiness to the adhesive after substantially all of thesolvent is removed from the adhesive coating by the dryers 17. Thetackiness of the adhesive coating after solvent removal is a verysignificant aspect of the invention in that it provides for wetting outof the metal foil substrate 12 when it is joined to the adhesive coatedpolyimide film web. The wetting out of the metal foil substrate 12 priorto the curing of the adhesive coating is believed to be a primary factorin the superior and advantageous high temperature integrity of thelamination 10.

It should be understood that in both the batch type and the continuoustype of operations, the adhesive composition should be formulated inaccordance with the invention to insure that the applied coating remainssomewhat tacky when the two plies of the lamination are united aftersubstantially all of the solvent is removed. The tackiness of theadhesive coating results in a wetting out of the substrate web withoutthe presence of solvents which might ultimately be detrimental to thefinal product.

In accordance with the invention, the curing of the epoxy resins of theadhesive formulation does not take place until after the adhesive issolvent coated onto the polyimide film material. Specifically, theadhesive mixture is maintained in an unreacted state until after it isin intimate contact with the second web material (preferably copperfoil) in the roller nip 16. The reaction between the acrylic copolymercomponent of the invention and the epoxy resins, i.e., the curing of theepoxy resins, is believed to take place by an epoxy amine mechanism.

The invention provides a method and an adhesive composition forproducing a polyimide film-metal foil lamination that has excellentresistance to high temperature delamination. The high temperatureintegrity of the new laminate is a result of using the new adhesiveformulation as a laminant and in producing the laminate in accordancewith the method of the invention. The ability for the new adhesive towet out the metal foil substrate and thereby permit solvent removalprior to the curing of the adhesive is of substantial significance inthe production of laminates particularly for those intended for end usesin printed circuit applications.

We claim:

1. A method of forming a laminate of a first web of polyimide film and asecond web, comprising the steps of a. supplying one of the webs to acoating station;

b. applying to said one web from a solvent solution an adhesive coatinghaving an ammoniated acrylic copolymer component and an epoxy component;

c. said epoxy component being present in a substantial excess inrelation to the quantity required to fully react with the acryliccopolymer;

d. removing substantially all the solvent from said adhesive film;

e. combining said adhesively coated web with the second web to form alamination; and

f. curing the adhesive coating by subjecting said lamination to elevatedtemperatures for a predetermined time.

2. The method of claim 1, wherein a. said curing step includes theapplication of heat and pressure simultaneously to the webs for a periodof about 5 minutes.

3. The method of claim 2, wherein a. said heat and pressure are appliedat about 450F.

4. The method of claim 1, wherein a. said elevated temperatures are fromabout 200F.

to about 260F.

5. The method of claim 4, wherein a. said predetermined time isapproximately 16 hours.

6. The method of forming a polyimide film-metal foil laminate whichincludes the steps of c. removing substantially all the solvent fromsaid adhesive film;

d. laminating the adhesive coated polyimide film to said metal foil;

e. said laminating step including the chemical reaction of epoxy groupsof said epoxy resin with the amine groups of said ammoniated acryliccopolymer.

7. The method of claim 6, wherein a. said chemical reaction is promotedby the exposure of the laminate to temperatures between about 200F. andabout 260F. for about 16 hours.

8. The method of claim 6, wherein a. said chemical reaction is promotedby the exposure of the laminate to a temperature of about 450F. forabout 5 minutes.

9. A laminate structure having high temperature integrity comprising a.a layer of polyimide film, adhered to a b. substrate of metal foil by ane. intermediate layer of a cured adhesive composition;

(1. said cured adhesive comprising the reaction product of approximatelyparts by weight of an ammoniated acrylic copolymer component and 46-100parts by weight of an epoxy resin component having an epoxide equivalentweight of less than about 550;

e. said adhesive composition having a substantial excess of epoxyreactive groups of at least 2.2 to l or greater for each reactive aminegroup of the said ammoniated acrylic copolymer component.

10. The laminate of claim 9, wherein a. said acrylic copolymer is acopolymer of methyl methacrylate and glycidyl methacrylate.

l l. The laminate of claim 10, wherein a. said epoxy resin includes alow molecular weight epichlorohydrin bisphenol A type epoxy compoundhaving an epoxide equivalent of about 190.

12. The laminate of claim 11, wherein a. said epoxy resin furtherincludes a small amount of a brominated epoxy compound having an epoxideequivalent weight of about 500.

13. A laminate structure having high temperature integrity comprising a.a layer of polyimide film, adhered to a b. substrate of metal foil by anc. intermediate layer of a cured adhesive composition;

d. said cured adhesive comprising the reaction product of an ammoniatedacrylic copolymer component and an epoxy resin component having anepoxide equivalent weight of less than about 550;

c. said epoxy resin component having an epoxy equivalent substantiallygreater than the amine equivalent of said ammoniated acrylic copolymercomponent.

III

1. A method of forming a laminate of a first web of polyimide film and asecond web, comprising the steps of a. supplying one of the webs to acoating stAtion; b. applying to said one web from a solvent solution anadhesive coating having an ammoniated acrylic copolymer component and anepoxy component; c. said epoxy component being present in a substantialexcess in relation to the quantity required to fully react with theacrylic copolymer; d. removing substantially all the solvent from saidadhesive film; e. combining said adhesively coated web with the secondweb to form a lamination; and f. curing the adhesive coating bysubjecting said lamination to elevated temperatures for a predeterminedtime.
 2. The method of claim 1, wherein a. said curing step includes theapplication of heat and pressure simultaneously to the webs for a periodof about 5 minutes.
 3. The method of claim 2, wherein a. said heat andpressure are applied at about 450*F.
 4. The method of claim 1, whereina. said elevated temperatures are from about 200*F. to about 260*F. 5.The method of claim 4, wherein a. said predetermined time isapproximately 16 hours.
 6. The method of forming a polyimide film-metalfoil laminate which includes the steps of a. supplying said polyimidefilm to a coating station; b. coating said polyimide web with anadhesive solvent solution containing a dry solids basis, 100 parts of anammoniated acrylic copolymer and 46-100 parts of an epoxy resin; c.removing substantially all the solvent from said adhesive film; d.laminating the adhesive coated polyimide film to said metal foil; e.said laminating step including the chemical reaction of epoxy groups ofsaid epoxy resin with the amine groups of said ammoniated acryliccopolymer.
 7. The method of claim 6, wherein a. said chemical reactionis promoted by the exposure of the laminate to temperatures betweenabout 200*F. and about 260*F. for about 16 hours.
 8. The method of claim6, wherein a. said chemical reaction is promoted by the exposure of thelaminate to a temperature of about 450*F. for about 5 minutes.
 9. Alaminate structure having high temperature integrity comprising a. alayer of polyimide film, adhered to a b. substrate of metal foil by anc. intermediate layer of a cured adhesive composition; d. said curedadhesive comprising the reaction product of approximately 100 parts byweight of an ammoniated acrylic copolymer component and 46-100 parts byweight of an epoxy resin component having an epoxide equivalent weightof less than about 550; e. said adhesive composition having asubstantial excess of epoxy reactive groups of at least 2.2 to 1 orgreater for each reactive amine group of the said ammoniated acryliccopolymer component.
 10. The laminate of claim 9, wherein a. saidacrylic copolymer is a copolymer of methyl methacrylate and glycidylmethacrylate.
 11. The laminate of claim 10, wherein a. said epoxy resinincludes a low molecular weight epichlorohydrin bisphenol A type epoxycompound having an epoxide equivalent of about
 190. 12. The laminate ofclaim 11, wherein a. said epoxy resin further includes a small amount ofa brominated epoxy compound having an epoxide equivalent weight of about500.